好像bfs也能吧

总体逻辑和dfs差不多

#include<bits/stdc++.h>
using namespace std;
#define pii pair<int,int>
int n,m,ans;
int a[110][110];
int xx[4] = {-1,0,1,0};
int yy[4] = {0,1,0,-1};
int bfs(int x,int y){
    int cnt=1;
	queue<pii>q;
    q.push({x,y});
    a[x][y] = 0;
    while(q.size()){
    	pii r = q.front();
        q.pop();
        for(int i=0;i<4;i++){
        	int nx = r.first+xx[i],ny = r.second+yy[i];
            if(nx<1||ny<1||nx>n||ny>m)continue;
            if(a[nx][ny]==0)continue;
            cnt++;
            a[nx][ny] = 0;
            q.push({nx,ny});
        }
    }
    return cnt;
}
int main(){
	cin >> n >> m;
    for(int i=1;i<=n;i++){
    	for(int j=1;j<=m;j++){
        	cin >> a[i][j];
        }
    }
    for(int i=1;i<=n;i++){
    	for(int j=1;j<=m;j++){
        	if(a[i][j]==1){
            	ans = max(ans,bfs(i,j));
            }
        }
    }
    cout << ans;
	return 0;
}